Anti-rat CD8a monoclonal antibody recognizes a determinant on the majority of thymocytes (90-95%), a subset of peripheral T cells, the majority of NK cells, and the granular intraepithelial leukocytes in the small intestine. (1,2,3,4.) The antigen recognized is a complex of surface glycoproteins of Mr 34,000, 39,000 and 76,000 Da and is the rat homologue of the human CD8 and the mouse Ly-2,3 antigen. (5,8) ABIN133188labels all peripheral T cells that are unlabelled by the ABIN133188(W3/25) monoclonal antibody. It labels a T-cell subset which mediates the suppression of antibody formation (1) and the cytotoxic cell precursor. (2) ABIN133188and ABIN133188can be used together to fractionate T cells by sorting in FACS or by rosette depletion (1,2) or can be used together to study subsets of T cells in the rat which mediate lethal graft vs. host disease. (7) This antibody is one of the 3 monoclonal antibodies which labels T lymphocyte populations in the rat, these being ABIN133188(W3/13) which labels all T cells, as well as ABIN133188(W3/25) and CL004F` (MRC 0X-8) which label non-overlapping T cell populations. These monoclonal antibodies used in concert are being employed extensively to investigate cellular aspects of the immune response in rats and prove to be useful as markers for functionally distinct subpopulations of lymphocytes.